Hydromagnesite or basic magnesium carbonate, which is the standard industrial name for hydromagnesite, is a naturally occurring mineral which is found in magnesium rich minerals such as serpentine and altered magnesium rich igneous rocks, but also as an alteration product of brucite in periclase marbles. Hydromagnesite is described as having the following formula:Mg5(CO3)4(OH)2.4H2O
It should be appreciated that hydromagnesite is a very specific mineral form of magnesium carbonate and occurs naturally as small needle-like crystals or crusts of acicular or bladed crystals. In addition thereto, it should be noted that hydromagnesite is a distinct and unique form of magnesium carbonate and is chemically, physically and structurally different from other forms of magnesium carbonate. Hydromagnesite can readily be distinguished from other magnesium carbonates by x-ray diffraction analysis, thermogravimetric analysis or elemental analysis. Unless specifically described as hydromagnesite, all other forms of magnesium carbonates (e.g. artinite (Mg2(CO3)(OH)2.3H2O), dypingite (Mg5(CO3)4(OH)2.5H2O), giorgiosite (Mg5(CO3)4(OH)2.5H2O), pokrovskite (Mg2(CO3)(OH)20.5H2O), magnesite (MgCO3), barringtonite (MgCO3.2H2O), lansfordite (MgCO3.5H2O) and nesquehonite (MgCO3.3H2O)) are not hydromagnesite within the meaning of the present invention and do not correspond chemically to the formula described above.
Besides the natural hydromagnesite, synthetic hydromagnesites (or precipitated magnesium carbonates) can be prepared. For instance, U.S. Pat. Nos. 1,361,324, 935,418, GB 548,197 and GB 544,907 generally describe the formation of aqueous solutions of magnesium bicarbonate (typically described as “Mg(HCO3)2”), which is then transformed by the action of a base, e.g., magnesium hydroxide, to form hydromagnesite. Other processes described in the art suggest to prepare compositions containing both, hydromagnesite and magnesium hydroxide, wherein magnesium hydroxide is mixed with water to form a suspension which is further contacted with carbon dioxide and an aqueous basic solution to form the corresponding mixture; cf. for example U.S. Pat. No. 5,979,461.
Additionally, general processes for preparing magnesium carbonate are described in the art. For example, EP 0 526 121 describes a calcium-magnesium carbonate composite consisting of calcium carbonate and magnesium carbonate hydroxide and a method for the preparation thereof. Furthermore, GB 594,262 relates to a method and apparatus for treating magnesia-containing materials, such as magnesium and calcium carbonate materials for obtaining respective carbonates in discrete and separate forms, by controlled carbonation such that the magnesium and calcium carbonates may be separated by mechanical means and with attainment of special utilities in separated products. US 2007194276 describes a method of reductively bleaching a mineral slurry comprising adding in the mineral slurry an effective amount of a formamidine sulfinic acid (FAS) and an effective amount of a borohydride to reductively bleach the mineral slurry.
In practice, hydromagnesite is used in huge quantities in the paper, rubber and plastics industries for various purposes such as coatings, fillers, extenders and pigments for papermaking as well as flame-retardants in electrical wires and cables but also to impart resistance to chemicals in fibers. For example, EP 0 543 262, EP 0 393 813, JP 21 50 436, JP 22 55 843, JP 51 70 984, JP 50 98 085 and KR 2003/0040953 describe flame-retardant compositions comprising hydromagnesite in admixture with other magnesium compounds such as huntite, dolomite and/or magnesium hydroxide. In this context, hydromagnesite in combination with various magnesium compounds is usually added into a resin composition for providing flame resistance and high mechanical strength so that such compositions can be used as a covering or insulation material for electric wires or cables, flame arresting materials, wall materials for various areas such as the automotive sector, for the production of housings for electrical appliances or in the building sector.
Another application for hydromagnesite is described in WO 2009/008600 which relates to a spandex fiber containing hydromagnesite and having resistance to chlorine without affecting intrinsic properties of the polyurethane polymer. Furthermore, WO 97/09473 describes spandex containing particles of a mineral mixture of huntite and hydromagnesite, wherein the spandex is described as having decreased tackiness and increased resistance to chlorine-induced degradation. Additionally, hydromagnesite in combination with other magnesium compounds is used in the paper industries in order to impart printability, a high brightness at high opacity but also suitable smoothness and gloss to paper products such as magazines. In this respect, JP 2003/293291 describes coated paper produced by disposing an adhesive and a coating layer consisting mainly of at least one of huntite and hydromagnesite on base paper, wherein the resulting coated paper has high brightness, a high surface-masking effect and excellent printing suitability.
Hydromagnesite and other magnesium compounds, e.g. magnesium carbonate and magnesium hydroxide, can also be incorporated as a filler in wrapping papers of smoking articles such as cigarettes or cigars in order to control many physical properties or characteristics such as the tar delivery per puff, burn rate, puff count, etc. One particularly important aspect of a smoking article that can be controlled by such wrapping paper is the sidestream smoke, which is the smoke given off by the burning end of a smoking article between puffs. However, as such smoke may be objectionable to other people near the smoker several attempts have been made to reduce such sidestream smoke through the use of various magnesium compounds. For example, U.S. Pat. No. 5,092,306 relates to a smoking article wrapper, and in particular, cigarette paper which uses magnesite as a filler composition. Others have used physical mixtures of magnesium hydroxide and hydromagnesite, e.g. U.S. Pat. Nos. 5,927,288 and 5,979,461, while others have made attempts for developing compositions wherein the amount of magnesium hydroxide is reduced by replacing this hydroxide with other magnesium compounds. For example, U.S. Pat. No. 5,253,660 discloses a cigarette or cigar wrapper wherein the paper filler consists of two homogeneously intermixed minerals, namely huntite and hydromagnesite, alone, or admixed with calcium carbonate or magnesium hydroxide or calcium carbonate and magnesium hydroxide and carbon.
However, with respect to the aforementioned possible applications of hydromagnesite, it is to be noted that there are significant constrains regarding the suitability of the corresponding filler particles or their application field. Hydromagnesite obtained from natural sources or prepared by processes described in the prior art for use as filler or coatings in paper applications, in smoking articles and/or as flame retardant usually has an average particle size of about 5 μm or more. In this context, it is to be noted that the thickness of, for example, wrapping papers for smoking articles is generally in the range of about 30 μm, so that the incorporation of hydromagnesite into smoking articles often do not impart the desired properties such as smoothness onto the surface of such articles and, thus, the physical and optical properties of the obtained products are not always satisfactory. Additionally, as well known from kaolin and its use as surface coating and filler in the field of paper applications, the morphology of the particles plays also a decisive role for imparting the desired optical and physical properties such as good printability, high brightness at high opacity, moderate porosity and a favorable smoothness and gloss into paper products such as magazines. For many applications, a platy-like morphology of the particles is highly favorable for obtaining said properties. The provision of platy-like particles having a small particle size would be especially advantageous. In this context, it is further to be noted that a mechanical comminution of particles is usually not a suitable and effective method for obtaining smaller particles of hydromagnesite.
Thus, there is still a need in the art for providing alternative processes for preparing hydromagnesite, wherein such process should be simple and inexpensive and should provide the possibility of controlling particular parameters such as the particle size in combination with the morphology and the density of the obtained particles.